PNG (Portable Network Graphics) Register and Extensions

The PNG Specification supports a backward-compatible
extension mechanism that need not involve changes to the core specification
itself. The documents on this page provide a registry and a specification
of such public extensions. (A similar mechanism exists for private extensions,
but, with few exceptions, they are not intended to become public and, in
any case, are not included here.)

The Register of PNG Public Chunks and Keywords contains a list of
all public chunks and text keywords, including their registration dates and
indication of where they are specified (i.e., the core PNG specification or
the PNG extensions document).

The Extensions to the PNG Specification (also often referred to as
the "PNG extensions document") contains a formal specification of those
chunks and text keywords that were either not deemed appropriate for
inclusion in the core PNG spec or have not yet completed the (lengthy)
process for such inclusion.

Note that Chapter 11 of
PNG: The Definitive Guide
complements the Register and the Extensions document by describing most
PNG chunks and keywords in a somewhat less formal, more expository manner.
(The remaining chunks are described in
Chapter 8 and
Chapter 10.) The book has no
formal standing, however, and in the case of discrepancies, the documents
below are considered definitive (as it were).